WAYNE, Mich. — During November in England, Ford will sell a diesel hatchback that gets 64 miles per gallon. Across the channel, Parisians can buy a new gas-powered compact made by General Motors that gets a nifty 47 mpg.

On these shores, neither carmaker sells anything that thrifty. Yet, with Americans clamoring for efficient cars and Detroit automakers on the ropes thanks to crashing sales of gas-guzzling trucks, the question is, why aren’t these vehicles here now?

Ford and GM say importing them from Europe isn’t an option because of unfavorable exchange rates. Instead, they’re racing to convert plants in America to produce them here. One of the first is the Michigan Truck Factory, home to the 14 mpg Lincoln Navigator and Ford Expedition.

Ford will shut down this half-century-old plant in November and begin the arduous process of converting it to a car factory where the European Ford Focus will be built in 2010. Ford is also preparing to retool two other truck factories to make five more European-style compact cars for the U.S. market.

“This is critical for us,” said Bill Russo, Ford’s head of manufacturing, walking past rows of roof assemblies in the Michigan Truck Plant’s body shop. “We absolutely need to make more smaller vehicles as soon as possible.”

And that’s a hugely ambitious logistical and engineering challenge.

To switch from trucks to cars, Ford will have to replace thousands of tools on the line, recali-brate a massive production process, establish new supply chains with hundreds of part makers and negotiate with the United Auto Workers union. Even with help coming in the form of recently approved $25 billion in federally backed loans, the financial burden will be tremendous.

The payoff is a chance to regain the market lost to imports like Toyota and Honda — in 2008, for the first time, less than half the cars sold in the U.S. were American brands. But if the Big Three are unable to deliver, experts say, the consequences could be disastrous.

“There is no going back to 2004,” said David Healey, auto industry analyst at Burnham Securities. “The American automakers need to make small cars, and they need to make profits on small cars, or they simply can’t continue.”

A complete makeover can cost up to $250 million per plant — or sometimes, according to Ron Harbour, auto industry consultant at Oliver Wyman Group, “driving a bulldozer from one side of the factory to the other, clearing everything out” at much greater expense.

But before powering down the auto robots, Detroit execs had to rethink the American market. For years, Ford and GM built high-end compact cars for Europe, routinely winning awards for Ford’s Mondeo and GM’s Vauxhall Corsa. The cars emphasized fit and finish, and offered options reserved for premium brands in the U.S. They cost more, but consumers happily paid because in Europe, Ford and GM are premium brands.

Until recently, the Big Three produced unprofitable small cars just to draw customers to dealerships, where they would often upgrade to highly profitable trucks and sport utility vehicles. When truck sales slipped, Detroit resisted bringing in European versions because they felt that the weak dollar and differing consumer expectations, not to mention the expense of altering them to comply with U.S. safety and emissions standards, made them untenable here.

“I wish I could bring the European Focus to the U.S.,” said Jim Farley, Ford’s head of sales and marketing, in January. “But it’ll never work in this market.”

But $4-a-gallon gas, plummeting truck sales and billions in quarterly losses changed Farley’s attitude. In July, Ford announced plans to bring six of its European cars across the pond. In August, GM said it would spend $500 million to bring the Cruze, a four-cylinder, 40-mpg compact targeted for Europe, to the U.S. About $350 million of that will retool the company’s Chevy Cobalt plant in Lordstown, Ohio, to make the Cruze, which should cost significantly more than the $15,850 Cobalt.

Meanwhile, GM is spending $500 million more on the Chevy Volt, a hybrid that will travel 40 miles on battery power alone and is stuffed with digital touch screens. It will be built at the company’s Hamtramck, Mich., plant, home of the boatlike Cadillac DTS and Buick Lucerne, neither of which tops 21 mpg. The Volt’s pricing hasn’t been finalized, but it could run as much as $40,000.

Chrysler, the smallest of the Big Three, is in a different situation. With virtually no international presence and a lineup that’s three-quarters trucks and SUVs, Chrysler doesn’t have many small cars on which to lose money. But with its U.S. sales down 25 percent through September compared with 2007, the company is trying myriad options to boost fuel efficiency.

All three carmakers hope more efficient cars will appeal to buyers. But they also need to make them to comply with fuel-efficiency regulations. Estimates on the total industry tab to reach the new 35-mpg standard by 2020 range from $80 billion to $100 billion.

To help defray costs, President Bush recently signed off on government guarantees for $25 billion in loans to carmakers and suppliers. They offer rates at less than one-third what the industry borrows on the open market, prompting some critics to call it a bailout. But insiders say it’s necessary.

“The new standards are a huge burden, so this is an essential source of capital,” said David Cole, chairman of the Center for Automotive Research, a nonprofit industry group.

Said Mark Warnsman, auto analyst at Calyon Securities: “The idea is to get American luxury car buyers to buy luxury small cars.”